With growing demand for higher storage capacities in optical recording systems new storage systems are being developed over CD (Compact Disc) and DVD (Digital Versatile Disc). The Blu-ray Disc (BD) is such a new storage system. Where a CD can store some 650 MB and the DVD some 4.7 GB per 12 cm diameter disc, the BD can already store some 27 GB on a 12 cm disc. For further increase in data storage capacity of a disc multi-layer media have been developed for DVD and also for BD. Such media contain, for example, two data layers, which are separated by a spacer layer. Each layer can be accessed through the same entrance surface of the record carrier.
The main differences from an optical point of view between these generations of optical disc systems are the wavelength of the radiation source and the numerical aperture of the objective system that focuses the radiation beam onto the data layer. For CD the wavelength is typically 785 nm and a numerical aperture (NA) of 0.45 is used, while for DVD this is about 660 nm and 0.60, and for BD about 405 nm and 0.85, respectively. To bridge all these different optical storage systems compatible optical scanning heads are being developed that are capable of reading and recording discs of the different optical recording system generations. When designing such compatible optical scanning heads it is desirable to use a single light-path and a single radiation detector as this leads to an optical head with fewer optical elements and, therefore, would be cheaper and more competitive.
Due to the difference in NA of the objective lens system for each application (CD, DVD or BD) also the NA of the returning radiation beam towards the radiation detector will be different. This implies that the diameter of the spot focused onto the photo-detector is also proportional to the NA. For example, a diameter of the radiation spot of 70 μm on the photo detector for a BD application with the astigmatic focusing method may use a typical photo-detector with dimensions of 200 μm by 100 μm, while the diameter of the radiation spot for the DVD application is about 54 μm and for the CD application only about 37 μm.
A problem arises when beamlanding occurs i.e. the detector and light spot are mutually displaced in the lateral direction due to for example mechanical stress, misalignment during manufacturing of the optical head or thermal instabilities in the optical scanning head. This beamlanding has deteriorating effects on the quality of the focusing and tracking signals of the scanning system. Beamlanding errors of up to 10-15 μm are common in optical scanning heads. For the DVD and CD application of the system such beamlanding values are relatively large with respect to the diameter of the radiation spot of respectively about 54 μm and 37 μm.
Another problem occurs when the system is reading dual-layer DVD discs. The radiation beam is focused by the objective system on one of the two information layers for reading out the data. The reflection at the other layer not being in focus causes a secondary spot on the radiation detector. This secondary spot has a much larger area on the radiation detector as it is out of focus at the record carrier, but a portion of the light distribution is still captured by the sensitive areas of the radiation detector. This secondary spot can be considered as unwanted stray light, as it may affect a robust focusing and tracking signal generation. In U.S. Pat. No. 5,841,735 the requirements for detector dimensions in relation to multi-layer disc are described. But the fixed detector dimensions optimized for the BD application, lead to a mismatch of such requirements and result in a large portion of stray light on the radiation detector for the dual-layer DVD application of the system that may result in reduction of the readout and recording performance of the system in the DVD application.
JP 10-177738 discloses a two-objective-lens actuator for a DVD-CD compatible optical scanning head with specific requirements on the relation between the numerical apertures and focal lengths of both objective lenses such that the numerical aperture of the radiation beam towards the radiation detector, thus the size of the focused spot on the radiation detector, is about the same for both the DVD as well as the CD application. When the optical scanning head also needs to be compatible with a third generation optical record carrier, such as BD, the proposed solution requires three lenses in the actuator. This will make the actuator head more complex and costly. It also increases the mass in the actuator, which limits the bandwidth of the actuator system, resulting in a limitation of the readout speed of record carriers. Also the possibility to use the solution in a 3D-actuator will become much more complex. A 3D-actuator is used for focusing and radial tracking of the record carrier as well as for the correction of record carrier tilt (usually in the radial direction). Such a 3D-actuator is commonly used in DVD systems.
It is an object of the invention to provide an optical scanning head (and its application) suitable for readout and/or recording of multiple record carrier generations without impact on the actuator performance and solving one or more of the above-mentioned problems.